Displays with Reduced Temperature Luminance Sensitivity

ABSTRACT

A display may include an array of pixels. Each pixel in the array includes an organic light-emitting diode coupled to a drive transistor, a data loading transistor, a first capacitor for storing data charge, and a second capacitor. During a data programming phase, the data loading transistor may be activated to load in a data value onto the first capacitor. After the data programming phase, the second capacitor may be configured to receive a lower voltage, which extends a threshold voltage sampling time for the pixel. Configured and operated in this way, the temperature luminance sensitivity of the display can be reduced.

This application is a continuation of patent application Ser. No.17/501,530, filed Oct. 14, 2021, which claims the benefit of provisionalpatent application No. 63/123,385, filed Dec. 9, 2020, which are herebyincorporated by reference herein in their entireties.

BACKGROUND

This relates generally to electronic devices with displays and, moreparticularly, to display driver circuitry for displays such as organiclight-emitting diode (OLED) displays.

Electronic devices often include displays. For example, cellulartelephones and portable computers typically include displays forpresenting image content to users. OLED displays have an array ofdisplay pixels based on light-emitting diodes. In this type of display,each display pixel includes a light-emitting diode and associatedthin-film transistors for controlling application of data signals to thelight-emitting diode to produce light. It can be challenging to design asatisfactory OLED display for an electronic device.

SUMMARY

An electronic device may include a display having an array of displaypixels. The display pixels may be organic light-emitting diode displaypixels. Each display pixel may include at least an organiclight-emitting diode (OLED) that emits light and associated thin-filmtransistors for controlling the operation of the pixel.

In accordance with some embodiments, a display is provided that includesgate driver circuitry and multiple pixels coupled to the gate drivercircuitry. At least one of the pixel can include: a drive transistorhaving a gate terminal, a first source-drain terminal, and a secondsource-drain terminal; a light-emitting diode having an anode coupled tothe second source-drain terminal of the drive transistor; a firstcapacitor having a first terminal coupled to the gate terminal of thedrive transistor and having a second terminal coupled to the anode; anda second capacitor having a first terminal coupled to the secondsource-drain terminal of the drive transistor and having a secondterminal configured to receive a control signal from the gate drivercircuitry. The gate driver circuitry can drive the control signal low onor after a data programming operation to extend a threshold voltagesampling time for the pixel.

The pixel can further include: a gate-to-drain transistor coupled acrossthe gate terminal and the first source-drain terminal of the drivetransistor; a data loading transistor having a first source-drainterminal coupled to the second source-drain terminal of the drivetransistor and having a second source-drain terminal coupled to a dataline; a first emission transistor having a first source-drain terminalcoupled to a positive power supply line and having a second source-drainterminal coupled to the first source-drain terminal of the drivetransistor; a second emission transistor having a first source-drainterminal coupled to the second source-drain terminal of the drivetransistor and having a second source-drain terminal coupled to theanode; and an initialization transistor having a first source-drainterminal coupled to the anode and having a second source-drain terminalcoupled to a voltage line.

In accordance with some embodiments, a method of operating a displaypixel is provided. The display pixel can include a light-emitting diode,a drive transistor coupled in series with the light-emitting diode, agate-to-drain transistor coupled across gate and drain terminals of thedrive transistor, a data loading transistor, a first capacitor coupledto the gate terminal of the drive transistor, and a second capacitorcoupled to a source terminal of the drive transistor. The method caninclude: during a data programming and threshold voltage sampling phase,using the data loading transistor to load data into the display pixelwhile the gate-to-drain transistor is activated; deactivating the dataloading transistor; and applying a control signal to the secondcapacitor to discharge the first capacitor after deactivating the dataloading transistor. The control signal can be generated using a gatedriver formed in the periphery of the pixel array. The control signalmay optionally be routed to the gate terminal of the data loadingtransistor. The method can further include performing an on-bias stressoperation before the data programming and threshold voltage samplingphase by activating the data loading transistor while the gate-to-draintransistor is deactivated.

In accordance with some embodiments, a display pixel is provided thatincludes: a substrate; a semiconducting oxide layer that is formed abovethe substrate and that forms an active region for a drive transistor,the drive transistor having a first source-drain terminal, a secondsource-drain terminal, and a gate terminal; a first metal layer formedabove the semiconducting oxide layer, the first metal layer having aportion that forms the gate terminal of the drive transistor and abottom terminal of a first capacitor; and a second metal layer formedabove the first metal layer, the second metal layer having a portionthat forms a top terminal of the first capacitor, wherein the secondsource-drain terminal of the drive transistor is coupled to a secondcapacitor, and wherein the second capacitor is configured to receive agate driver signal.

The second capacitor can have a bottom terminal formed from anotherportion of the first metal layer and can have a top terminal formed fromanother portion of the second metal layer. The display pixel can alsoinclude a source-drain metal routing layer formed above the second metallayer and optionally a third metal layer formed between the substrateand the semiconducting oxide layer. The third metal layer can be coupledto the second source-drain terminal of the drive transistor. The secondcapacitor can have a bottom terminal formed from a portion of the thirdmetal layer, the first metal layer, or the second metal layer and canhave a top terminal formed from a portion of the first metal layer, thesecond metal layer, or the source-drain metal routing layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an illustrative electronic device having adisplay in accordance with some embodiments.

FIG. 2 is a diagram of an illustrative display having an array oforganic light-emitting diode display pixels in accordance with someembodiments.

FIG. 3 is a diagram illustrating a sampling current path during athreshold voltage sampling phase in accordance with some embodiments.

FIG. 4A is a timing diagram showing how the gate-to-source voltage of adisplay pixel drive transistor can vary in accordance with someembodiments.

FIG. 4B is a timing diagram showing how a sampling current can vary inaccordance with some embodiments.

FIG. 5 is a diagram illustrating temperature luminance sensitivityprofiles at different sampling current levels in accordance with someembodiments.

FIG. 6A is a circuit diagram of an illustrative display pixel configuredto reduce temperature luminance sensitivity in accordance with someembodiments.

FIG. 6B is a timing diagram illustrating a data programming andthreshold voltage sampling phase and an extended threshold voltagesampling phase in accordance with some embodiments.

FIG. 7A is a circuit diagram of an illustrative display pixel having adata loading transistor and a threshold voltage sampling extensioncapacitor driven using separate peripheral gate drivers in accordancewith some embodiments.

FIG. 7B is a circuit diagram of an illustrative display pixel having adata loading transistor and a threshold voltage sampling extensioncapacitor driven using a shared peripheral gate driver in accordancewith some embodiments.

FIG. 7C is a circuit diagram of an illustrative display pixel having adata loading transistor and a threshold voltage sampling extensioncapacitor connected within the pixel and driven using a peripheral gatedriver in accordance with some embodiments.

FIG. 8A is a circuit diagram of an illustrative display pixel having atleast three semiconducting oxide transistors in accordance with someembodiments.

FIG. 8B is a timing diagram showing illustrative waveforms involved inoperating the display pixel of FIG. 8A in accordance with someembodiments.

FIG. 9A is a circuit diagram of an illustrative display pixel having atleast four semiconducting oxide transistors in accordance with someembodiments.

FIG. 9B is a timing diagram showing illustrative waveforms involved inoperating the display pixel of FIG. 9A in accordance with someembodiments.

FIG. 10A is a circuit diagram of an illustrative display pixel having adata loading transistor and a threshold voltage sampling extensioncapacitor shorted together in accordance with some embodiments.

FIG. 10B is a circuit diagram of an illustrative display pixel operableto perform an extended threshold voltage sampling phase in accordancewith some embodiments.

FIG. 11A is a circuit diagram of an illustrative display pixel having atleast five semiconducting oxide transistors in accordance with someembodiments.

FIG. 11B is a timing diagram showing illustrative waveforms involved inoperating the display pixel of FIG. 11A in accordance with someembodiments.

FIG. 12A is a circuit diagram of an illustrative display pixel having atleast six semiconducting oxide transistors in accordance with someembodiments.

FIGS. 12B and 12C are timing diagrams showing illustrative waveformsinvolved in operating the display pixel of FIG. 12A in accordance withsome embodiments.

FIG. 13A is a circuit diagram of an illustrative display pixel having adata loading transistor and a threshold voltage sampling extensioncapacitor separately driven by peripheral gate drivers in accordancewith some embodiments.

FIG. 13B is a timing diagram showing illustrative waveforms involved inoperating the display pixel of FIG. 13A in accordance with someembodiments.

FIG. 14A-14E are cross-sectional side views of a display stackup showingat least a drive transistor, a storage capacitor, and a thresholdvoltage sampling extension capacitor in accordance with someembodiments.

FIGS. 15A and 15B are circuit diagrams of an illustrative display pixelhaving a p-type drive transistor that is coupled to a light-emittingdiode having a common cathode terminal in accordance with someembodiments.

FIGS. 16A and 16B are circuit diagrams of an illustrative display pixelhaving an n-type drive transistor that is coupled to a light-emittingdiode having a common anode terminal in accordance with someembodiments.

FIG. 17 is a circuit diagram of an illustrative display pixel having ap-type drive transistor that is coupled to a light-emitting diode havinga common anode terminal in accordance with some embodiments.

DETAILED DESCRIPTION

An illustrative electronic device of the type that may be provided witha display is shown in FIG. 1 . As shown in FIG. 1 , electronic device 10may have control circuitry 16. Control circuitry 16 may include storageand processing circuitry for supporting the operation of device 10. Thestorage and processing circuitry may include storage such as hard diskdrive storage, nonvolatile memory (e.g., flash memory or otherelectrically-programmable-read-only memory configured to form a solidstate drive), volatile memory (e.g., static or dynamicrandom-access-memory), etc. Processing circuitry in control circuitry 16may be used to control the operation of device 10. The processingcircuitry may be based on one or more microprocessors, applicationprocessors, microcontrollers, digital signal processors, basebandprocessors, power management units, audio chips, application specificintegrated circuits, etc.

Input-output circuitry in device 10 such as input-output devices 12 maybe used to allow data to be supplied to device 10 and to allow data tobe provided from device 10 to external devices. Input-output devices 12may include buttons, joysticks, scrolling wheels, touch pads, key pads,keyboards, microphones, speakers, tone generators, vibrators, cameras,sensors, light-emitting diodes and other status indicators, data ports,etc. A user can control the operation of device 10 by supplying commandsthrough input-output devices 12 and may receive status information andother output from device 10 using the output resources of input-outputdevices 12.

Input-output devices 12 may include one or more displays such as display14. Display 14 may be a touch screen display that includes a touchsensor for gathering touch input from a user or display 14 may beinsensitive to touch. A touch sensor for display 14 may be based on anarray of capacitive touch sensor electrodes, acoustic touch sensorstructures, resistive touch components, force-based touch sensorstructures, a light-based touch sensor, or other suitable touch sensorarrangements.

Control circuitry 16 may be used to run software on device 10 such asoperating system code and applications. During operation of device 10,the software running on control circuitry 16 may display images ondisplay 14 using an array of pixels in display 14. Device 10 may be atablet computer, laptop computer, a desktop computer, a display, acellular telephone, a media player, a wristwatch device or otherwearable electronic equipment, or other suitable electronic device.

Display 14 may be an organic light-emitting diode display or may be adisplay based on other types of display technology. Configurations inwhich display 14 is an organic light-emitting diode (OLED) display aresometimes described herein as an example. This is, however, merelyillustrative. Any suitable type of display may be used in device 10, ifdesired.

Display 14 may have a rectangular shape (i.e., display 14 may have arectangular footprint and a rectangular peripheral edge that runs aroundthe rectangular footprint) or may have other suitable shapes. Display 14may be planar or may have a curved profile.

A top view of a portion of display 14 is shown in FIG. 2 . As shown inFIG. 2 , display 14 may have an array of pixels 22 formed on a substrate36. Substrate 36 may be formed from glass, metal, plastic, ceramic,porcelain, or other substrate materials. Pixels 22 may receive datasignals over signal paths such as data lines D (sometimes referred to asdata signal lines, column lines, etc.) and may receive one or morecontrol signals over control signal paths such as horizontal controllines G (sometimes referred to as gate lines, scan lines, emissionlines, row lines, etc.). There may be any suitable number of rows andcolumns of pixels 22 in display 14 (e.g., tens or more, hundreds ormore, or thousands or more).

Each pixel 22 may have a light-emitting diode 26 that emits light 24under the control of a pixel control circuit formed from thin-filmtransistor circuitry such as thin-film transistors 28 and thin-filmcapacitors). Thin-film transistors 28 may be polysilicon thin-filmtransistors, semiconducting oxide thin-film transistors such as indiumzinc gallium oxide transistors, or thin-film transistors formed fromother semiconductors. Pixels 22 may contain light-emitting diodes ofdifferent colors (e.g., red, green, and blue) to provide display 14 withthe ability to display color images.

Display driver circuitry 30 may be used to control the operation ofpixels 22. The display driver circuitry 30 may be formed from integratedcircuits, thin-film transistor circuits, or other suitable electroniccircuitry. Display driver circuitry 30 of FIG. 2 may containcommunications circuitry for communicating with system control circuitrysuch as control circuitry 16 of FIG. 1 over path 32. Path 32 may beformed from traces on a flexible printed circuit or other cable. Duringoperation, the control circuitry (e.g., control circuitry 16 of FIG. 1 )may supply circuitry 30 with information on images to be displayed ondisplay 14.

To display the images on display pixels 22, display driver circuitry 30may supply image data to data lines D (e.g., data lines that run downthe columns of pixels 22) while issuing clock signals and other controlsignals to supporting display driver circuitry such as gate drivercircuitry 34 over path 38. If desired, display driver circuitry 30 mayalso supply clock signals and other control signals to gate drivercircuitry 34 on an opposing edge of display 14 (e.g., the gate drivercircuitry may be formed on more than one side of the display pixelarray).

Gate driver circuitry 34 (sometimes referred to as horizontal linecontrol circuitry or row driver circuitry) may be implemented as part ofan integrated circuit and/or may be implemented using thin-filmtransistor circuitry. Horizontal/row control lines G in display 14 maycarry gate line signals (scan line control signals), emission enablecontrol signals, and/or other horizontal control signals for controllingthe pixels of each row. There may be any suitable number of horizontalcontrol signals per row of pixels 22 (e.g., one or more row controllines, two or more row control lines, three or more row control lines,four or more row control lines, five or more row control lines, etc.).

FIG. 3 is a diagram showing a portion of pixel 22. As shown in FIG. 3 ,pixel 22 may include at least a drive transistor such as transistorTdrive, a storage capacitor such as capacitor Cst, a first switch suchas switch Tgd, and a second switch such as switch Tdata. Drivetransistor Tdrive is configured to provide a drive current to diode 26(see FIG. 2 ) and has a gate (G) terminal, a drain (D) terminal, and asource (S) terminal. The terms “source” and “drain” terminals that areused to describe current-conducting terminals of a transistor aresometimes interchangeable and may be referred to herein as“source-drain” terminals. Storage capacitor Cst may be coupled to thegate terminal of transistor Tdrive. Switch Tgd (e.g., a thin-filmtransistor such as an n-type semiconducting-oxide transistor, an n-typesilicon transistor, or a p-type silicon transistor) is coupled acrossthe drain and gate terminals of transistor Tdrive and is thereforesometimes referred to as a gate-to-drain transistor. Switch Tdata (e.g.,a thin-film transistor such as an n-type semiconducting-oxidetransistor, an n-type silicon transistor, or a p-type silicontransistor) is coupled between the source terminal of transistor Tdriveand a data line D and is therefore sometimes referred to as a dataloading transistor.

In practice, pixel 22 may be subject to process, voltage, andtemperature (PVT) variations. Due to such variations, transistorthreshold voltages between different display pixels 22 can vary.Variations in the threshold voltage of the drive transistor can causedifferent display pixels 22 to produce amounts of light that do notmatch the desired image. In an effort to mitigate threshold voltagevariations, display pixel 22 of the type shown in FIG. 3 may be operableto support in-pixel threshold voltage (Vth) compensation. In-pixelthreshold voltage compensation operations, sometimes referred to asin-pixel Vth canceling operations, may generally include at least aninitialization phase, a data programming and Vth sampling phase, and anemission phase. During the threshold voltage sampling phase, thethreshold voltage of transistor Tdrive may be sampled using storagecapacitor Cst. Subsequently, during the emission phase, an emissioncurrent flowing from transistor Tdrive into the light-emitting diode 26has a term that cancels out with the sampled Vth. As a result, theemission current will be independent of the drive transistor thresholdvoltage Vth and will therefore be immune to any Vth variations at thedrive transistor. During the data programming and Vth sampling phase, acurrent can flow through switch Tgd, transistor Tdrive, and switchTdata, as indicated by sampling current path Isample.

FIG. 4A is a timing diagram showing how the gate-to-source voltage Vgsof transistor Tdrive can vary during the data programming and Vthsampling phase. As shown by curve 50 in FIG. 4A, Vgs may have an initialvoltage level of Vgs(0) at the beginning of the Vth sampling phase (attime t0) and may gradually discharge toward the threshold voltage levelVth. In practice, the time period for the Vth sampling phase is oftenconstrained by the row access time, which means that Vth has to besampled within a relatively short amount of time at sampling timet_sample. Terminating the Vth sampling phase at time t_sample may causepixel 22 to sample a voltage that is ΔV above Vth, where ΔV represents aVth sampling residue amount. It is generally desirable to minimize theVth sampling residue ΔV.

FIG. 4B is a timing diagram showing how the drive current flowingthrough transistor Tdrive can vary during the data programming and Vthsampling phase. As shown by curve 52 in FIG. 4B, the drive current Idsmay also begin decreasing at the beginning of the Vth sampling phase (attime t0). Terminating the Vth sampling phase at time t_sample willresult in a final current level of Isample flowing through the drivetransistor.

The sampling current level Isample may affect a display's sensitivity totemperature. FIG. 5 is a diagram illustrating temperature luminancesensitivity profiles (plotting temperature luminance sensitivity versusgray level) at different sampling current levels. Temperature luminancesensitivity may be proportional to the change in luminance in responseto a predetermined change in temperature. It is generally desirable tokeep the temperature luminance sensitivity as close to zero as possibleto minimize the display's sensitivity to temperature.

As shown in FIG. 5A, curve 60 plots the temperature luminancesensitivity profile for a pixel having a first Isample level, whereascurve 62 plots the temperature luminance sensitivity profile for a pixelhaving a second Isample level that is lower than the first Isamplelevel. Especially at lower gray levels, curve 62 has a temperatureluminance sensitivity level S2 that is closer to zero than curve 60,which has a temperature luminance sensitivity level S1. A largernegative temperature luminance sensitivity induced at lower gray levelscan result in display non-uniformity that is visible to the human eye.Thus, operating a pixel at lower Isample levels can help provide atechnical improvement to the display by reducing temperature luminancesensitivity. For example, the required voltage swing at the gate ofTdrive to change the absolute value of the drain current through Tdrivefrom 1 pA to 10 pA at a lower temperature is larger than that at ahigher temperature. Referring back to FIG. 4B, reducing Isample requiresincreasing or pushing out the sampling time t_sample. In conventionaldisplay pixel architectures, the Vth sampling duration is, however,limited by the duration of the data programming period (i.e., the dataprogramming period is typically limited to one row time, which is set bythe performance requirements of the display).

In accordance with an embodiment, FIG. 6A is a circuit diagram ofillustrative display pixel 22 configured to reduce temperature luminancesensitivity by extending the threshold voltage sampling period beyondthe data programming phase. As shown in FIG. 6A, display pixel 22 mayinclude a light-emitting element such as an organic light-emitting diode26, a capacitor such as storage capacitor Cst, and thin-film transistorssuch a drive transistor Tdrive, a gate-to-drain transistor Tgd, a dataloading switch (transistor) Tdata, an initialization switch (transistor)Tini, and emission switches (transistors) Tem1 and Tem2. At least someor all of the transistors/switches within pixel 22 such as Tdrive, Tgd,Tdata, Tini, Tem1, and Tem2 are semiconducting oxide transistors.Semiconducting oxide transistors are defined as thin-film transistorshaving a channel region formed from semiconducting oxide material (e.g.,indium gallium zinc oxide or IGZO, indium tin zinc oxide or ITZO, indiumgallium tin zinc oxide or IGTZO, indium tin oxide or ITO, or othersemiconducting oxide material) and are generally considered n-type(n-channel) transistors.

A semiconducting oxide transistor is notably different than a silicontransistor (i.e., a transistor having a polysilicon channel regiondeposited using a low temperature process sometimes referred to as LTPSor low-temperature polysilicon). Semiconducting oxide transistorsexhibit lower leakage than silicon transistors, so implementing at leastsome of the transistors within pixel 22 can help reduce flicker (e.g.,by preventing current from leaking away from the gate terminal of drivetransistor Tdrive).

If desired, at least some of the transistors within pixel 22 may beimplemented as silicon transistors such that pixel 22 has a hybridconfiguration that includes a combination of semiconducting oxidetransistors and silicon transistors (e.g., n-type LTPS transistors orp-type LTPS transistors). In yet other suitable embodiments, pixel 22may include one or more anode reset transistors configured to reset theanode (A) terminal of diode 26. As another example, display pixel 22 mayfurther include one or more initialization transistors for apply aninitialization or reference voltage to an internal node within pixel 22.As another example, display pixel 22 may further include additionalswitching transistors (e.g., one or more additional semiconducting oxidetransistors or silicon transistors) for applying one or more biasvoltages for improving the performance or operation of pixel 22.

Drive transistor Tdrive has a gate terminal G, a drain terminal D(sometimes referred to as a first source-drain terminal), and a sourceterminal S (sometimes referred to as a second source-drain terminal).Drive transistor Tdrive, emission control transistors Tem1 and Tem2, andlight-emitting diode 26 are coupled in series between positive powersupply line 600 and ground power supply line 602. Emission transistorTem1 has a gate terminal configured to receive a first emission controlsignal EM1, whereas emission transistor Tem2 has a gate terminalconfigured to receive a second emission control signal EM2. This examplein which transistors Tem1 and Tem2 receive two different emissionsignals is merely illustrative. As another example, transistors Tem1 andTem2 can receive the same emission control signal.

A positive power supply voltage VDDEL may be supplied to positive powersupply terminal 600, whereas a ground power supply voltage VSSEL may besupplied to ground power supply terminal 602. Positive power supplyvoltage VDD may be 3 V, 4 V, 5 V, 6 V, 7 V, 2 to 8 V, greater than 6 V,greater than 8 V, greater than 10 V, greater than 12 V, 6-12 V, 12-20 V,or any suitable positive power supply voltage level. Ground power supplyvoltage VSSEL may be 0 V, −1 V, −2 V, −3 V, −4 V, −5 V, −6V, −7 V, lessthan 2 V, less than 1 V, less than 0 V, or any suitable ground ornegative power supply voltage level. During emission operations, signalsEM1 and EM2 are asserted to turn on transistors Tem1 and Tem2, whichallows current to flow from drive transistor Tdrive to diode 26. Thedegree to which drive transistor Tdrive is turned on controls the amountof current flowing from terminal 600 to terminal 602 through diode 26and therefore the amount of emitted light from display pixel 22.

In the example of FIG. 6A, storage capacitor Cst may be coupled betweenthe gate terminal of drive transistor Tdrive and the anode (A) terminalof diode 26. Transistor Tgd may have a first source-drain terminalconnected to the gate terminal of transistor Tdrive, a secondsource-drain terminal connected to the drain terminal of drivetransistor Tdrive, and a gate terminal configured to receive a firstscan control signal SC1. Data loading transistor Tdata may have a firstsource-drain terminal connected to the source terminal of transistorTdrive, a second source-drain terminal connected to the data line, and agate terminal configured to receive a second scan control signal SC2.Scan control signals SC1, SC2, and SC3 may be provided over row controllines (see lines G in FIG. 2 ). Transistor Tini may have a firstsource-drain terminal connected to the anode terminal (sometimesreferred to as the anode electrode) of diode 26, a second source-drainterminal configured to receive an initialization (reference) voltageVini via an initialization voltage line, and a gate terminal configuredto receive a third scan control signal SC3. Initialization voltage Vinican also sometimes be referred to as an anode reset voltage Var. Diode26 has a cathode terminal (sometimes referred to as the cathodeelectrode) coupled to VSSEL ground power supply line 602 (sometimesreferred to as the common power supply line).

In particular, display pixel 22 may further include a capacitor such ascapacitor Cx having a first terminal coupled to the source terminal oftransistor Tdrive and a second terminal configured to receive a controlsignal X. Control signal X may be generated by a gate driver circuit andmay therefore sometimes be referred to as a gate driver signal. Controlsignal X may be adjusted in a way that extends the threshold voltagesampling time beyond the data programming phase. FIG. 6B is a timingdiagram illustrating the operation of display pixel 22 of the type shownin FIG. 6A. At time t1, scan signal SC1 may be asserted (driven high) toturn on (activate) transistor Tgd. At time t2, scan signal SC2 may bepulsed high to temporarily activate data loading transistor Tdata. Whilescan signal SC2 is high from time t2 to t3, transistor Tdata isconfigured to load in a data signal from the data line onto sourceterminal S of the drive transistor. This time period during which bothtransistors Tdata and Tgd are activated is sometimes referred to as thedata programming (loading) and Vth sampling phase. The gate-to-sourcevoltage of the drive transistor Vgs will initially jump up at time t2and will slowly discharge during the data programming and Vth samplingphase.

At time t3, data loading transistor Tdata is turned off (deactivated),which terminates the data programming phase. If no further action istaken, Vgs will hold its current value (see voltage level 70) since thecharge on capacitor Cst has nowhere is discharge and the Vth samplingphase will also terminate. At time t3, however, signal X may toggle froma first voltage level to a second voltage level that is less than thefirst voltage level. Lowering signal X in this way will initially causeVgs to rise at time t3, but then current will start flowing fromcapacitor Cst to capacitor Cx through the drive transistor. This currentpath from Cst to Cx will cause Vgs to continue to decrease as long asscan signal SC1 is asserted. A Vgs that continues to decrease belowvoltage level 70 even after transistor Tdata has been turned offeffectively extends the threshold voltage sampling time since thevoltage held on capacitor Cst will continue to update or dischargeitself to a value that is closer to the true Vth level, therebyminimizing the Vth sampling residual value ΔV (see FIG. 4A).

The time period from time t3 (when Tdata is deactivated) to time t4(when Tgd is deactivated) during which Vth sampling can continue to takeplace even after the data programming phase has terminated may thereforesometimes be referred to as an extended threshold voltage (Vth) samplingphase. Capacitor Cx that is used to extend the Vth sampling period maytherefore sometimes be referred to as a threshold voltage samplingextension capacitor. As described in connection with FIGS. 4B and 5 , alonger sampling time can result in lower Isampling levels, whichultimately reduces a display's temperature luminance sensitivity. Theexample of FIG. 6B in which the lowering of signal X is synchronizedwith the deassertion of scan signal SC2 is merely illustrative. Ifdesired, the signal X adjustment may be delayed to time t3′ (see dottedwaveform), which can occur at any time after time t3 (i.e., any timeafter Tdata is deactivated) and before time t4 (i.e., any time beforeTgd is deactivated). Configured and operated in this way, the displaywill be less sensitive to temperature variations and will thereforeexhibit improved thermal uniformity.

In general, the scan control signals are routed using separate scanlines. For example, scan signal SC1 may be generated using a first gatedriver circuit and routed to pixel 22 via a first scan (row) line, scansignal SC2 may be generated using a second gate driver circuit androuted to pixel 22 via a second scan (row) line, and scan signal SC3 maybe generated using a third gate driver circuit and routed to pixel 22via a third scan (row) line. Scan control signal SC2 and capacitorbiasing signal X may or may not be generated using the same gate driverwithin gate driver circuitry 34 (FIG. 2 ).

FIG. 7A illustrates a first embodiment in which scan signal SC2 isgenerated using a first gate driver 35-1 within gate driver circuitry34, whereas capacitor biasing signal X is generated using a second gatedriver 35-2 within gate driver circuitry 34. In other words, signals SC2and X are generated using separate dedicated gate drivers in theperiphery of the display pixel array and are fed to transistor Tdata andcapacitor Cx via respective row lines. In the example of FIG. 7A,transistors Tdrive and Tgd may be implemented as semiconducting oxidetransistors. The remaining transistors such as transistors Tdata, Tini,Tem1, Tem2, and/or other switches within pixel 22 can each beimplemented as a semiconducting oxide transistor or a silicon transistor(e.g., an n-type LTPS transistor or a p-type LTPS transistor).

FIG. 7B illustrates another embodiment in which scan signal SC2 andcapacitor biasing signal X are generated using the same gate driver 35within gate driver circuitry 34. As shown in FIG. 7B, the output of gatedriver 35 may be routed to the gate of transistor Tdata via a first rowline and may be routed to capacitor X via a second row line differentthan the first row line. In this arrangement, signal X will have thesame waveform as signal SC2 (e.g., signal X will be deasserted at thesame time as SC2). In the example of FIG. 7B, transistor Tdata may alsobe implemented as a semiconducting oxide transistor. The remainingtransistors such as transistors Tini, Tem1, Tem2, and/or other switcheswithin pixel 22 can each be implemented as a semiconducting oxidetransistor or a silicon transistor (e.g., an n-type LTPS transistor or ap-type LTPS transistor). In the example of FIG. 7B where scan signal SC1and SC2 have the same polarity (i.e., both SC1 and SC2 are driven highto turn on transistors Tgd and Tdata, respectively), signal X can bedriven using the same gate driver that generates scan signal SC2.

FIG. 7C illustrates yet another embodiment in which scan signal SC2 andcapacitor biasing signal X are generated using the same gate driver 35within gate driver circuitry 34. As shown in FIG. 7C, the gate terminalof transistor Tdata is directly coupled to capacitor Cx via a wire 700within pixel 22 (e.g., the gate of Tdata and the bottom terminal of Cxare shorted internally within pixel 22). Connected in this way, theoutput of gate driver 35 is routed to the gate of transistor Tdata viaonly one row line (instead of using two different row lines as shown inthe example of FIG. 7B). In this arrangement, signal X will have thesame waveform as signal SC2 (e.g., signal X will be deasserted at thesame time as SC2). In the example of FIG. 7C where scan signal SC1 andSC2 have the same polarity (i.e., both SC1 and SC2 are driven high toturn on transistors Tgd and Tdata, respectively), signal X can be drivenusing the same gate driver that generates scan signal SC2.

FIG. 8A illustrates another example in which pixel 22 includes threesemiconducting oxide transistors. As shown in FIG. 8A, transistorsTdrive, Tgd, and Tini may be implemented as semiconducting oxidetransistors, whereas transistors Tdata, Tem1, and Tem2 are implementedas p-type silicon transistors. Here, scan signal SC2(n) and capacitorbiasing signal X(n) are provided via separate row lines similar to theexample of FIG. 7A where signals SC2 and X are generated using separateperipheral gate drivers. The notation “(n)” refers to the row that pixel22 belongs too. Thus, transistor Tem2 will receive an emission signalEM(n) from an emission driver in the same row as pixel 22, whereastransistor Tem1 will receive an emission signal EM(n+2) that is routedfrom another emission driver configured to drive pixels two rows belowpixel 22. Note that in the example of FIG. 8A, the initializationtransistor Tini is also controlled by emission signal EM(n) (e.g., thegate terminals of transistors Tini and Tem2 may be shorted together).

FIG. 8B is a timing diagram showing illustrative waveforms involved inoperating the display pixel of FIG. 8A. Prior to time t1 when both EM(n)and EM(n+2) are asserted (e.g., driven high for the p-channel emissiontransistors), pixel 22 may operate in the emission phase. Signal EM(n+2)may be a delayed version of signal EM(n). When signal EM(n) isdeasserted (e.g., driven high), the emission phase terminates.

At time t1 (at the beginning of an initialization phase), control signalSC1(n) is pulsed high to activate transistor Tgd. Since signal EM(n+2)is still low at this time, transistor Tem1 is activated. Since bothtransistors Tem1 and Tgd are on, the gate and drain terminals of thedrive transistor will be pulled up to positive power supply voltageVDDEL. Since signal EM(n) is high, transistor Tini will drive the anodeelectrode of diode 26 to the Vini voltage level. This period cansometimes be referred to as an “anode reset” phase. Storage capacitorCst is coupled across the gate terminal of Tdrive and the anodeterminal. During the initialization phase, the voltage across capacitorCst is therefore reset to a predetermined voltage difference(VDDEL-Vini). Signal SC1(n) is deasserted at time t2, which marks theend of the initialization and anode reset phase. Signal EM(n+2) issubsequently driven high some time after t2 and before t3, which turnsoff transistor Tem1.

At time t3, scan signal SC(2) is pulsed low to temporarily activate thedata loading transistor Tdata. Turning on transistor Tdata will load adata voltage Vdata onto the source terminal of the drive transistor suchthat the voltage Vs at the source terminal of Tdrive is set to Vdata(i.e., Vs=Vdata). Scan signal SC1(n) is low during this time, whichkeeps transistor Tgd deactivated. As a result, the voltage the gate ofthe drive transistor cannot change. In certain situations, thresholdvoltage Vth can shift, such as when display 14 is transitioning from ablack image to a white image or when transitioning from one gray levelto another. This shifting in Vth (sometimes referred to herein asthin-film transistor “hysteresis”) can cause a reduction in luminance,which is otherwise known as “first frame dimming.”

For example, the saturation current Ids waveform as a function of Vgs ofthe drive transistor for a black frame might be slightly offset from thetarget Ids waveform as a function of Vgs of the drive transistor for awhite frame. Without performing an on-bias stress operation, the sampledVth will correspond to the black frame and will therefore deviate fromthe target Ids waveform by quite a large margin. By performing on-biasstress, the sampled Vth will correspond to Vdata and will therefore bemuch closer to the target Ids curve. Performing the on-bias stress phaseto bias the Vgs of the drive transistor with Vdata before sampling Vthcan therefore help mitigate hysteresis and improve first frame response.An “on-bias stress phase” may therefore be defined as an operation thatapplies a suitable bias voltage directly to the drive transistor duringnon-emission phases (e.g., such as by turning on the data loadingtransistor Tdata). The on-bias stress phase terminals at time t4 whenscan signal SC1(n) is driven high.

At time t4, scan signal SC1(n) is driven high to reactivategate-to-drain transistor Tgd. From time t4 to t5, transistors Tgd andTdata are both activated. Activating transistor Tdata will load datasignal D(n) into pixel 22 (e.g., by driving the data signal onto thesource terminal of transistor Tdrive). Since signal SC1(n) is high, thevoltage at the gate and drain terminals of transistor Tdrive will shiftup or down depending on the value of D(n) while retaining a Vthdifference across the gate and source terminals since the voltage hasnowhere to discharge. The time period from time t4 to t5 is thereforesometimes referred to as a data programming and Vth sampling phase. Thedata programming period may be equal to or less than one row time.

At time t5, scan signal SC2(n) is driven high, which deactivatestransistor Tdata and terminates the data programming operation. Sometime between t5 and t6, signal X(n) is driven low. As described above inconnection with FIG. 6B, driving signal X(n) low after the dataprogramming phase can help extend the Vth sampling time by dischargingcurrent from capacitor Cst to capacitor Cx via transistor Tgd. The timeperiod between time t5 (when the data programming phase terminates) andtime t6 (when transistor Tgd is deactivated) may therefore sometimes bereferred to as the extended Vth sampling phase. The falling edge of scansignal SC1(n) can be adjusted to tune the duration of the extended Vthsampling period. At time t7, emission control signals EM(n) and EM(n+2)are both asserted (driven low) to resume the emission period.

The example of FIG. 8A in which the data loading transistor Tdata isimplemented as a p-channel silicon transistor is merely illustrative.FIG. 9A illustrates another example where the data loading transistorTdata is implemented as a semiconducting oxide transistor. As shown inFIG. 9A, pixel 22 now includes at least four semiconducting oxidetransistors (e.g., transistors Tdrive, Tgd, Tini, and Tdata may all besemiconducting oxide switches). The remainder of pixel 22 is similar toFIG. 8A and need not be reiterated in detail to avoid obscuring thepresent embodiment. FIG. 9B is a timing diagram showing illustrativewaveforms involved in operating the display pixel of FIG. 9A. Theoperation illustrated in FIG. 9B is similar to that already shown inFIG. 8B, except scan signal SC2(n) of FIG. 9B is inverted with respectto scan signal SC2(n) of FIG. 8B to control the n-channel semiconductingoxide transistor Tdata.

The example of FIG. 9A in which SC2(n) and X(n) are connected todifferent row lines is merely illustrative. FIG. 10A shows anotherexample where capacitor Cx is directly connected to the gate oftransistor Tdata, similar to the configuration of FIG. 7C. The remainderof pixel 22 is similar to FIG. 9A and need not be reiterated in detailto avoid obscuring the present embodiment. The timing diagram foroperating pixel 22 of FIG. 10A is similar to the timing diagram of FIG.9B without the X(n) waveform. Since capacitor Cx is shorted to the gateof Tdata, the X(n) waveform will be identical to that of scan signalSC2(n).

The example of FIG. 10A in which capacitor Cx is coupled between thegate and source terminals of transistor Tdata is merely illustrative.FIG. 10B shows another implementation where capacitor Cx can optionallybe left out from pixel 22 of FIG. 10A if the parasitic gate-to-sourcecapacitance of semiconducting oxide transistor Tdata is large enough toprovide sufficient capacitive coupling from the SC2(n) signal to thesource terminal of the drive transistor. The size of transistor Tdatacan be increased relative to the other transistors in pixel 22 toobviate the need to form capacitor Cx. For example, transistor Tdata canbe larger than each of the emission transistors, the initializationtransistor, Tdrive, Tgd, and/or other switching transistors in pixel 22.The timing diagram for operating pixel 22 of FIG. 10B is similar to thetiming diagram of FIG. 9B but without the X(n) waveform.

FIG. 11A illustrates another example in which pixel 22 includes fivesemiconducting oxide transistors. As shown in FIG. 11A, transistorsTdrive, Tgd, Tdata, Tem1, and Tem2 may be implemented as semiconductingoxide transistors, whereas transistor Tini is implemented as a p-typesilicon transistor. Here, capacitor Cx is shorted to the gate oftransistor Tdata within pixel 22 similar to the example of FIG. 7C wheresignals SC2 and X are generated using the same peripheral gate driver.

FIG. 11B is a timing diagram showing illustrative waveforms involved inoperating the display pixel of FIG. 11A. Prior to time t1 when bothEM(n) and EM(n+2) are asserted (e.g., driven high for the n-channelemission transistors), pixel 22 may operate in the emission phase.Signal EM(n+2) may be a delayed version of signal EM(n). When signalEM(n) is deasserted (e.g., driven low), the emission phase terminates.

At time t1 (at the beginning of an initialization phase), control signalSC1(n) is pulsed high to activate transistor Tgd. Since signal EM(n+2)is still high at this time, transistor Tem1 is activated. Since bothtransistors Tem1 and Tgd are on, the gate and drain terminals of thedrive transistor will be pulled up to positive power supply voltageVDDEL. Since signal EM(n) is low, transistor Tini will drive the anodeterminal of diode 26 to the Vini voltage level. This period cansometimes be referred to as the anode reset phase. Storage capacitor Cstis coupled across the gate terminal of Tdrive and the anode terminal.During the initialization phase, the voltage across capacitor Cst istherefore reset to a predetermined voltage difference (VDDEL−Vini).Signal SC1(n) is deasserted at time t2, which marks the end of theinitialization and anode reset phase. Signal EM(n+2) is subsequentlydriven low some time after t2 and before t3, which turns off transistorTem1.

At time t3, scan signal SC(2) is pulsed low to temporarily activate thedata loading transistor Tdata during the on-bias stress phase. Turningon transistor Tdata will load a data voltage Vdata onto the sourceterminal of the drive transistor such that the voltage Vs at the sourceterminal of Tdrive is set to Vdata (i.e., Vs=Vdata). Scan signal SC1(n)is low during this time, which keeps transistor Tgd deactivated. As aresult, the voltage the gate of the drive transistor cannot change. Byperforming on-bias stress, a later sampled Vth will correspond to Vdataand will therefore be much closer to the target Ids curve. Performingthe on-bias stress phase to bias the Vgs of the drive transistor withVdata before sampling Vth can therefore help mitigate hysteresis andimprove first frame response. The on-bias stress phase terminals at timet4 when scan signal SC1(n) is driven high.

At time t4, scan signal SC1(n) is driven high to reactivategate-to-drain transistor Tgd. From time t4 to t5, transistors Tgd andTdata are both activated. Activating transistor Tdata will load datasignal D(n) into pixel 22 (e.g., by driving the data signal onto thesource terminal of transistor Tdrive). Since signal SC1(n) is high, thevoltage at the gate and drain terminals of transistor Tdrive will shiftup or down depending on the value of D(n) while retaining a Vthdifference across the gate and source terminals since the voltage hasnowhere to discharge. The time period from time t4 to t5 is thereforesometimes referred to as a data programming and Vth sampling phase. Thedata programming period may be equal to or less than one row time.

At time t5, scan signal SC2(n) is driven low, which deactivatestransistor Tdata and terminates the data programming operation. Drivingscan signal SC2(n) low will simultaneously apply a lower voltage tocapacitor Cx. As described above in connection with FIG. 6B, supplying alower voltage to capacitor Cx after the data programming phase can helpextend the Vth sampling time by discharging current from capacitor Cstto capacitor Cx via transistor Tgd. The time period between time t5(when the data programming phase terminates) and time t6 (whentransistor Tgd is deactivated) may therefore sometimes be referred to asthe extended Vth sampling phase. The falling edge of scan signal SC1(n)can be adjusted to tune the duration of the extended Vth samplingperiod. At time t7, emission control signals EM(n) and EM(n+2) are bothasserted (driven high) to resume the emission period.

The example of FIG. 11A in which the initialization transistor Tini isimplemented as a p-channel silicon transistor is merely illustrative.FIG. 12A illustrates another example where the initialization transistorTini is implemented as a semiconducting oxide transistor. As shown inFIG. 12A, pixel 22 now includes at least six semiconducting oxidetransistors (e.g., transistors Tdrive, Tgd, Tini, Tdata, Tem1, and Tem2may all be semiconducting oxide switches). Pixel 22 of FIG. 12A does notinclude any silicon transistors. In particular, transistor Tini may nowbe controlled by emission signal EMB(n), which is an inverted version ofsignal EM(n). The remainder of pixel 22 is similar to FIG. 11A and neednot be reiterated in detail to avoid obscuring the present embodiment.

FIG. 12B is a timing diagram showing illustrative waveforms involved inoperating the display pixel of FIG. 12A. The operation illustrated inFIG. 12B is similar to that already shown in FIG. 11B, except an extrasignal EMB(n) is required to control transistor Tini. The example ofFIG. 12B in which scan signal SC2(n) is pulsed high at time t3 toperform the on-bias stress operation is merely illustrative. FIG. 12C isa timing diagram illustrating another example where scan signal SC1(n)is continuously asserted (driven high) from time t1 until time t5.Operated in this way, there will be no on-bias stress operation prior totime t3.

The example of FIG. 12A in which transistor Tdata and capacitor Cx bothreceive scan signal SC2(n) is merely illustrative. FIG. 13A illustratesanother example where the data loading transistor Tdata and Cx receiveseparate signals SC2(n) and X(n), respectively, via different gatedrivers. As shown in FIG. 13A, pixel 22 includes at least sixsemiconducting oxide transistors (e.g., all of the transistors withinpixel 22 are semiconducting oxide switches). The remainder of pixel 22is similar to FIG. 12A and need not be reiterated in detail to avoidobscuring the present embodiment. FIG. 13B is a timing diagram showingillustrative waveforms involved in operating the display pixel of FIG.13A. The operation illustrated in FIG. 13B is similar to that alreadyshown in FIG. 12B with an additional signal X(n) that is different thansignal SC2(n). As shown in FIG. 13B, signal X(n) may be driven higharound time t1 and is driven low on or after time t4 but before time t5.

FIG. 14A is a cross-sectional side view of a display pixel 22 having afirst storage capacitor Cst and a second Vth sampling extensioncapacitor Cx (see, e.g., illustrative pixels 22 of FIGS. 6-13 ). Asshown in FIG. 14A, the display may have a stackup that includes asubstrate layer such as substrate 100. Substrate 100 may optionally becovered with one or more buffer layers 102. Buffer layer(s) 102 mayinclude inorganic buffer layers such as layers of silicon oxide, siliconnitride, or other passivation or dielectric material.

A semiconducting oxide layer 104 may be formed on buffer layer 102. Asemiconducting oxide layer is defined as an oxide layer that is formedfrom a semiconductor such as IGZO, IGTZO, ITO, ITZO, or othersemiconductor material. Oxide layer 104 may be patterned to formrespective channel portions of semiconducting oxide transistors such astransistor Tdrive. A gate insulating layer such as layer 106 may beformed over oxide layer 104. Gate insulating layer 106 may be formedfrom silicon oxide, silicon nitride, silicon oxynitride, tantalum oxide,cerium oxide, carbon-doped oxide, aluminum oxide, hafnium oxide,titanium oxide, vanadium oxide, spin-on organic polymeric dielectrics,spin-on silicon based polymeric dielectric, a combination of thesematerials, and other suitable low-k or high-k solid insulating material.

A top gate conductive layer such as gate layer G may be formed on gateinsulating layer 106. Top gate conductors G may be formed frommolybdenum, titanium, aluminum, nickel, chromium, copper, silver, gold,a combination of these materials, other metals, or other suitable gateconductor material. In the example of FIG. 5 , semiconducting oxidelayer 104 and a portion of the gate conductor layer directly above layer104 collectively forms transistor Tdrive (as an example).

A first interlayer dielectric (ILD) layer 108 may be formed over gateconductor G. A second gate conductor layer such as gate layer G′ may beformed on layer 108. Gate conductor G′ may also be formed frommolybdenum, titanium, aluminum, nickel, chromium, copper, silver, gold,a combination of these materials, other metals, or other suitable gateconductor material. A second interlayer dielectric (ILD) layer 110 maybe formed over gate conductor G′.

A first source-drain metal routing layer SD1 may be formed on layer 110.The SD1 metal routing layer may be formed from aluminum, nickel,chromium, copper, molybdenum, titanium, silver, gold, a combination ofthese materials (e.g., a multilayer stackup of Ti/Al/Ti), other metals,or other suitable metal routing conductors. The SD1 metal routing layermay be patterned and/or etch to form SD1 metal routing paths.

In the example of FIG. 14A, capacitor Cst may be formed directly abovetransistor Tdrive. In particular, capacitor Cst may have a bottomcapacitor plate formed from a portion of the first gate conductor layerG and a top capacitor plate formed from a portion of the second gateconductor layer G′. Layers G and G″ may sometimes be referred to asfirst and second metal layers, respectively. Capacitor Cx may be formedlateral to capacitor Cst. As shown in FIG. 14A, capacitor Cx may have abottom capacitor plate formed from another portion of the first gateconductor layer G and a top capacitor plate formed from another portionof the second gate conductor layer G′. The top capacitor terminal of Cxmay be coupled to the source terminal (S) of the drive transistor viaSD1 routing.

FIG. 14B shows another embodiment having a backside conductor G″ formedunderneath the semiconducting oxide layer 104. As shown in FIG. 14B,backside conductor G″ may be formed on substrate 100 and under bufferlayer 102. Conductor G″ may be formed using molybdenum, aluminum,nickel, chromium, copper, titanium, silver, gold, a combination of thesematerials, other metals, or other suitable conductive material.Conductor G″ may therefore sometimes be referred to as a third metallayer. Conductor G″ may be configured as a shielding layer to blockfringing electric fields from adjacent nodes in the pixel (e.g., toshield the backside channel from potentially interfering electricfield). If desired, conductor G″ may also serve as a backside gateconductor for the drive transistor. Conductor G″ may be shorted to thesource terminal of the drive transistor via the SD1 metal routing toreduce pixel crosstalk and to reduce potential non-uniformity issues.

The example of FIG. 14A in which capacitor Cx is formed using metallayers G and G′ is merely illustrative. FIG. 14C shows anotherembodiment where capacitor Cx is formed using other layers in thedisplay stackup. As shown in FIG. 14C, capacitor Cx may have a bottomcapacitor plate formed from a portion of metal layer G′ and a topcapacitor plate formed from a portion of the SD1 metal layer. Theremainder of FIG. 14B is substantially similar to FIG. 14A and need notbe reiterated in detail to avoid obscuring the present embodiment.

The example of FIG. 14B in which capacitor Cx is formed using metallayers G and G′ is merely illustrative. FIG. 14D shows anotherembodiment where capacitor Cx is formed using other layers in thedisplay stackup. As shown in FIG. 14D, capacitor Cx may have a bottomcapacitor plate formed from a portion of metal layer G′ and a topcapacitor plate formed from a portion of the SD1 metal layer. Theremainder of FIG. 14D is substantially similar to FIG. 14B and need notbe reiterated in detail to avoid obscuring the present embodiment.

The example of FIG. 14D in which backside conductor G″ is formeddirectly below the drive transistor is merely illustrative. FIG. 14Eshows another embodiment where the backside conductor G″ extends beyondthe drive transistor. As shown in FIG. 14E, capacitor Cx may have abottom capacitor plate formed using the extended backside conductor G″and a top capacitor plate formed from a portion of metal layer G. Inother words, a first portion of backside conductor layer G″ serves as abottom shield/gate for the drive transistor, whereas a second portion ofbackside conductor layer G′ serves as the bottom plate for capacitor Cx.The remainder of FIG. 14E is substantially similar to FIG. 14D and neednot be reiterated in detail to avoid obscuring the present embodiment.If desired, the top plate of capacitor Cx can instead be formed usingsecond metal layer G′ or the SD1 metal layer.

The example of pixel 22 in FIG. 6A where the drive transistor is ann-type (n-channel) transistor and where diode 26 has a cathode terminalcoupled to the VSSEL power supply line is merely illustrative. FIG. 15Aillustrates another embodiment where display pixel 22 includes a p-type(p-channel) drive transistor that is coupled to diode 26 having a commoncathode terminal (i.e., diode 26 has a cathode electrode coupled to thecommon VSSEL ground power supply line). As shown in FIG. 15A, at leastdrive transistor Tdrive and data loading transistor Tdata may besemiconducting oxide transistors. Capacitor Cst may have a firstterminal coupled to the gate terminal of transistor Tdrive and a secondterminal coupled to the VDDEL power supply line.

Pixel 22 may include a first initialization switch (transistor) Tini1having a first source-drain terminal coupled to the gate terminal oftransistor Tdrive and a second source-drain terminal coupled to a firstinitialization line configured to receive a first initialization voltageVini1. Pixel 22 may also include a second initialization switch(transistor) Tini2 having a first source-drain terminal coupled to theanode electrode of diode 26 and a second source-drain terminal coupledto a second initialization line configured to receive a secondinitialization voltage Vini2. Initialization transistors Tini1 and Tini2may be controlled using scan control signals SC4 and SC3, respectively.Pixel 22 may include a first emission switch (transistor) Tem1 coupledin series between the anode electrode and the drain terminal oftransistor Tdrive and may include a second emission switch (transistor)Tem2 coupled in series between the source terminal of transistor Tdriveand the VDDEL power supply line.

Transistor Tdata and capacitor Cx are coupled to the source terminal ofthe drive transistor. Although transistors Tdata and capacitor Cx areshown as being separately driven by gate drivers 35-1 and 35-2,respectively, signals X and SC2 can be driven using the same gate driver(see, e.g., FIGS. 7B and 7C) if scan signals SC1 and SC2 have the samepolarity (i.e., both SC1 and SC2 are driven high or low to turn ontransistors Tgd and Tdata, respectively). In general, switches Tem1,Tem2, Tini1, Tini2, and/or Tdata can each be implemented as asemiconducting oxide transistor, an n-channel silicon transistor, or ap-channel silicon transistor.

The embodiment of FIG. 15A in which the first initialization transistorTini1 is coupled to the gate terminal of transistor Tdrive is merelyillustrative. FIG. 15B shows another embodiment where initializationtransistor Tini1 has a first source-drain terminal coupled to the drainterminal of transistor Tdrive, has a second source-drain terminalconfigured to receive voltage Vini1, and has a gate terminal configuredto receive scan signal SC4. The remainder of pixel 22 has a structuresimilar to that already described in connection with FIG. 15A and neednot be reiterated in detail to avoid obscuring the present embodiment.

The example of pixel 22 in FIGS. 15A and 15B where diode 26 has acathode terminal coupled to the VSSEL power supply line is merelyillustrative. FIG. 16A illustrates another embodiment where displaypixel 22 includes an n-type drive transistor that is coupled to diode 26having a common anode terminal (i.e., diode 26 has an anode electrodecoupled to the common VDDEL positive power supply line). As shown inFIG. 16A, at least drive transistor Tdrive and data loading transistorTdata may be semiconducting oxide transistors. Capacitor Cst may have afirst terminal coupled to the gate terminal of transistor Tdrive and asecond terminal coupled to the VSSEL ground power supply line.

Pixel 22 may include a first initialization switch (transistor) Tini1having a first source-drain terminal coupled to the gate terminal oftransistor Tdrive and a second source-drain terminal coupled to a firstinitialization line configured to receive a first initialization voltageVini1. Pixel 22 may also include a second initialization switch(transistor) Tini2 having a first source-drain terminal coupled to thecathode electrode of diode 26 and a second source-drain terminal coupledto a second initialization line configured to receive a secondinitialization voltage Vini2. Initialization transistors Tini1 and Tini2may be controlled using scan control signals SC4 and SC3, respectively.Pixel 22 may include a first emission switch (transistor) Tem1 coupledin series between the cathode electrode and the drain terminal oftransistor Tdrive and may include a second emission switch (transistor)Tem2 coupled in series between the source terminal of transistor Tdriveand the VSSEL power supply line.

Transistor Tdata and capacitor Cx are coupled to the source terminal ofthe drive transistor. Although transistors Tdata and capacitor Cx areshown as being separately driven by gate drivers 35-1 and 35-2,respectively, signals X and SC2 can be driven using the same gate driver(see, e.g., FIGS. 7B and 7C) if scan signals SC1 and SC2 have the samepolarity (i.e., both SC1 and SC2 are driven high or low to turn ontransistors Tgd and Tdata, respectively). In general, switches Tem1,Tem2, Tini1, Tini2, and/or Tdata can each be implemented as asemiconducting oxide transistor, an n-channel silicon transistor, or ap-channel silicon transistor.

The embodiment of FIG. 16A in which the first initialization transistorTini1 is coupled to the gate terminal of transistor Tdrive is merelyillustrative. FIG. 16B shows yet another embodiment where initializationtransistor Tini1 has a first source-drain terminal coupled to the drainterminal of transistor Tdrive, has a second source-drain terminalconfigured to receive voltage Vini1, and has a gate terminal configuredto receive scan signal SC4. The remainder of pixel 22 has a structuresimilar to that already described in connection with FIG. 16A and neednot be reiterated in detail to avoid obscuring the present embodiment.

The example of pixel 22 in FIGS. 16A and 16B where the drive transistoris an n-type transistor is merely illustrative. FIG. 17 illustrates yetanother embodiment where display pixel 22 includes a p-type drivetransistor that is coupled to diode 26 having a common anode terminal(i.e., diode 26 has an anode electrode coupled to the common VDDELpositive power supply line). As shown in FIG. 17 , at least data loadingtransistor Tdata may be a semiconducting oxide transistor. Capacitor Cstmay have a first terminal coupled to the gate terminal of transistorTdrive and a second terminal coupled to the cathode terminal.

Pixel 22 may include an initialization switch (transistor) Tini having afirst source-drain terminal coupled to the cathode electrode and asecond source-drain terminal coupled to an initialization lineconfigured to receive initialization voltage Vini. Pixel 22 mayoptionally include one or more additional initialization transistorscoupled to the cathode terminal or some other internal node within pixel22. Initialization transistor Tini may be controlled using scan controlsignal SC3. Pixel 22 may include a first emission switch (transistor)Tem1 coupled in series between the VSSEL power supply line and the drainterminal of Tdrive and may include a second emission switch (transistor)Tem2 coupled in series between the source terminal of transistor Tdriveand the cathode electrode.

Transistor Tdata and capacitor Cx are coupled to the source terminal ofthe drive transistor. Although transistors Tdata and capacitor Cx areshown as being separately driven by gate drivers 35-1 and 35-2,respectively, signals X and SC2 can be driven using the same gate driver(see, e.g., FIGS. 7B and 7C) if scan signals SC1 and SC2 have the samepolarity (i.e., both SC1 and SC2 are driven high or low to turn ontransistors Tgd and Tdata, respectively). In general, transistorsTdrive, Tem1, Tem2, Tini, and/or Tdata can each be implemented as asemiconducting oxide transistor, an n-channel silicon transistor, or ap-channel silicon transistor.

The foregoing is merely illustrative and various modifications can bemade to the described embodiments. The foregoing embodiments may beimplemented individually or in any combination.

What is claimed is:
 1. A display pixel comprising: a light-emittingdiode; a drive transistor coupled in series with the light-emittingdiode; a first capacitor having a first terminal coupled to a gateterminal of the drive transistor and having a second terminal coupled toan anode of the light-emitting diode; and a second capacitor having afirst terminal coupled to a source terminal of the drive transistor andhaving a second terminal configured to receive an adjustable voltagesignal.
 2. The display pixel of claim 1, further comprising: a dataloading transistor coupled to the source terminal of the drivetransistor and configured to receive the adjustable voltage signal. 3.The display pixel of claim 2, further comprising: a gate driver havingan output coupled to the second terminal of the second capacitor andcoupled to a gate terminal of the data loading transistor.
 4. Thedisplay pixel of claim 3, further comprising: a first emissiontransistor coupled between a power supply line and a drain terminal ofthe drive transistor; a second emission transistor coupled between thesource terminal of the drive transistor and the anode of thelight-emitting diode; a gate-to-drain transistor coupled between thedrain terminal and the gate terminal of the drive transistor; and aninitialization transistor having a first terminal coupled to the anodeof the light-emitting diode and a second terminal coupled to aninitialization line.
 5. The display pixel of claim 4, wherein the drivetransistor comprises an n-type silicon transistor and wherein the dataloading transistor comprises a semiconducting oxide transistor.
 6. Thedisplay pixel of claim 5, wherein the gate-to-drain transistor comprisesan n-type silicon transistor.
 7. The display pixel of claim 1, furthercomprising: a data loading transistor coupled to the source terminal ofthe drive transistor and configured to receive a scan control signalseparate from the adjustable voltage signal.
 8. The display pixel ofclaim 7, further comprising: a first gate driver having an outputcoupled to a gate terminal of the data loading transistor; and a secondgate driver having an output coupled to the second terminal of thesecond capacitor.
 9. The display pixel of claim 8, further comprising: afirst emission transistor coupled between a power supply line and adrain terminal of the drive transistor; a second emission transistorcoupled between the source terminal of the drive transistor and theanode of the light-emitting diode; a gate-to-drain transistor coupledbetween the drain terminal and the gate terminal of the drivetransistor; and an initialization transistor having a first terminalcoupled to the anode of the light-emitting diode and a second terminalcoupled to an initialization line.
 10. The display pixel of claim 9,wherein: the drive transistor, the gate-to-drain transistor, the dataloading transistor, and the initialization transistor comprisesemiconducting oxide transistors; and the first emission transistor andthe second emission transistor comprise p-type silicon transistors. 11.The display pixel of claim 9, wherein the second emission transistor isconfigured to receive an emission signal and wherein the initializationtransistor is configured to receive the emission signal.
 12. The displaypixel of claim 11, wherein the first emission transistor is configuredto receive an additional emission signal from another display pixel in adifferent row.
 13. The display pixel of claim 1, further comprising: adata loading transistor coupled to the source terminal of the drivetransistor and configured to receive the adjustable voltage signal; afirst emission transistor coupled between a power supply line and adrain terminal of the drive transistor; a second emission transistorcoupled between the source terminal of the drive transistor and theanode of the light-emitting diode; a gate-to-drain transistor coupledbetween the drain terminal and the gate terminal of the drivetransistor; and an initialization transistor having a first terminalcoupled to the anode of the light-emitting diode and a second terminalcoupled to an initialization line.
 14. The display pixel of claim 13,wherein: the drive transistor, the gate-to-drain transistor, the dataloading transistor, and the initialization transistor comprisesemiconducting oxide transistors; and the first emission transistor andthe second emission transistor comprise p-type silicon transistors. 15.The display pixel of claim 1, further comprising: a data loadingtransistor coupled to the source terminal of the drive transistor andconfigured to receive the adjustable voltage signal via a first rowline, wherein the second capacitor is configured to receive theadjustable voltage signal via a second row line separate from the firstrow line.
 16. The display pixel of claim 1, further comprising: a dataloading transistor coupled to the source terminal of the drivetransistor and configured to receive the adjustable voltage signal via arow line, wherein the second capacitor is configured to receive theadjustable voltage signal via the row line.
 17. A method of operating adisplay pixel having a light-emitting diode, a drive transistor, and afirst capacitor coupled between a gate terminal of the drive transistorand an anode of the light-emitting diode, the method comprising: duringan initialization phase, asserting a first scan signal to activate agate-to-drain transistor coupled across the gate terminal and a drainterminal of the drive transistor; during the initialization phase,driving a control voltage from a first voltage level to a second voltagelevel, the control voltage being applied to a second capacitor coupledto a source terminal of the drive transistor; and after a dataprogramming phase and while the first scan signal is asserted, drivingthe voltage from the second voltage level to the first voltage level.18. The method of claim 17, further comprising: after the initializationphase, asserting the first scan signal to start the data programmingphase; and during the data programming phase, asserting a second scansignal to activate a data loading transistor.
 19. The method of claim18, further comprising: after the initialization phase and before thedata programming phase, asserting the second scan signal to start anon-bias stress phase.
 20. A display pixel comprising: a light-emittingdiode; a drive transistor coupled in series with the light-emittingdiode; a first emission transistor coupled between a power supply lineand a first source-drain terminal of the drive transistor; a secondemission transistor coupled between a second source-drain terminal ofthe drive transistor and an anode of the light-emitting diode andconfigured to receive an emission signal; a gate-to-drain transistorcoupled between a gate terminal and the first source-drain terminal ofthe drive transistor; a data loading transistor coupled to the secondsource-drain terminal of the drive transistor; an initializationtransistor coupled to the anode terminal of the light-emitting diode andconfigured to receive the emission signal; a first capacitor having afirst terminal coupled to the gate terminal of the drive transistor andhaving a second terminal coupled to the anode of the light-emittingdiode; and a second capacitor having a first terminal coupled to thesecond source-drain terminal of the drive transistor and having a secondterminal configured to receive a control voltage via a row line.